The invention relates to fixed wing aircraft propellers and, more particularly, to method and apparatus for increasing propeller efficiency through beamwise flapping. Propeller blades of the prior art have been curved, oriented, flexed and angulated by a myriad of methods and apparatus addressed toward improving propeller efficiency. In conventional systems, the propeller blades are individually mounted for varying pitch control relative to rotation. Other designs have similarly been implemented to improve performance. One such prior art method and apparatus is set forth in U.S. Pat. No. 425,692 issued to D'Aubarede on June 1, 1943. The D'Aubarede patent teaches elastic propeller blade mountings for avoiding or reducing the transmission of vibrations resulting from periodic torques from the power shaft to the propeller blades. It has been recognized to be an advantage to give the propeller limited amounts of freedom in tilting its tip-path plane with respect to the power shaft axis. Such "maneuvering" avoids the stresses resulting from the propeller blades passing in front of a wing or any other obstruction to the flow of air. Similarly a lack of balance between the blades, and gyroscopic torques caused by the oscillations of the power shaft axis are stress contribution factors and equally elements of concern. It may be seen, however that elastic mountings permit the propeller blade to oscillate in any direction with respect to the power shaft axis. This subject invention notes however, that beamwise deflection rather than radial, in-plane or torsional deflection is the key advantage of blade flexibility. Torsional flexibility may actually decrease propeller blade efficiencies in periods of maximum loading such as during take-off and high-speed cruise. The provision of high mechanical strength in the torsional and/or radial plane is thus necessary for maximum utilization of propulsive capacity of any system design.
Prior art approaches to propeller assemblies have also addressed hub mounted, flexural elements and propeller blade-shaft combinations. Most often emphasis on the variable-pitch, cyclic control aspect of propeller/rotor driven aircraft has been toward helicopters rather than fixed wing aircraft. Such applications of the aforesaid assemblies have always been used for flight control and to relieve structural loads in the propeller blades, hub and shafts. This invention uses these elements to increase propeller thrust and thereby increase propulsive efficiency. It has been found to be desirable to provide blade tilt assemblies for fixed wing aircraft to maximize and/or direct propulsive thrust. In such a manner, the propulsive effect of the propeller may be selectively utilized to meet certain flight conditions. For example, lateral and vertical deflection of propeller wake by the utilization of "vanes" is set forth in U.S. Pat. No. 1,289,343 issued to A. Wolff, Jr. on Dec. 31, 1918. In the Wolff patent, a plurality of vanes have been arranged at an angle to the longitudinal axis of the airplane to operate in conjunction with the propellers to produce a select forward pressure. In the Wolff structure currents from the propellers strike the vanes at an angle and thereby exert a pressure which ostensibly accelerates the forward movement of the airplane.
It may be seen that directing propeller wakes and tilting propellers is thus not a new concept. Various "wake" characteristics of propeller driven aircraft have been considered by the prior art. Most conventional design considerations are addressed, in the main, by variable pitch propeller blades. However, lift-drag coefficients play an important part of increasing any propulsive efficiency. The angle of attack of a rotating propeller blade is, in particular, a critical aspect of efficiency and overall performance. Moreover, a small increase in propeller efficiency of but a few percent will reduce fuel consumption of the aircraft, which in the present energy shortage, is a most important consideration in aircraft design. Modern fixed wing aircraft must, therefore, take such design aspects into consideration.
A variety of functional mechanisms have been set forth in the prior art for orienting and angulating propeller blades of both fixed and rotational wing aircraft, for flight control. In the main, the gyroscopic, accelerative, and drag effects of the propeller blades have been of tantamount concern. U.S. Pat. No. 3,799,695 issued to Eiichi Yamakawa on Mar. 26, 1974 sets forth a rotor control system having variable pitch rotor blades and a swashplate system which is controlled by the pilot's control stick. The amount of longitudinal cyclic pitch change induced by the longitudinal control stick operation is decreased as the forward flight speed increases and concurrently the lateral cyclic pitch change is automatically reduced.
Cyclic pitch control of propeller blades has thus been effected, and comprises an important aspect for control of rotary wing aircraft.
Concomitantly, certain aspects of propeller blade flapping have been implemented as shown in disclosures addressing helicopter flight. U.S. Pat. No. 2,509,313 issued to C. G. Pullin on May 30, 1950 sets forth such a method of achieving the advantage of a large ratio of pitch change for small flapping displacements of the propeller blades. The Pullin patent sets forth a gimbal ring and offset flapping hinge for rotary wing aircraft. The advantages of such flapping and propeller wing control have not, however, been particularly utilized to date for fixed wing aircraft.
Fixed wing, multi-propeller aircraft of conventional design are generally certified to achieve a specified rate of climb at a specified climb speed with one propeller inoperative. This is particularly critical for aircraft with only two conventional propellers installed. Once one propeller has failed, there is only one operative thrust developing means for propelling the aircraft. Such a propeller has two distinct areas in its tip-path plane (propeller disc). The first region is the down-side area in which the blades rotate substantially downward through a horizontal plane (drawn through the center of the propeller hub). The second region is the up-side area in which the blades rotate substantially upwards through the horizontal plane. For conventional propellers in flight with their axis of rotation at typical angles of 5 to 15 degrees above the vector of the free stream velocity, and when flying at the specified climb speed, the majority of the propeller thrust is produced on the down side of the propeller disk. On the up side, the blades are producing much less than their design capability. It may thus be seen that the propeller blades are experiencing a "cyclical change" in an angle of attack and relative airspeed during each hub revolution. Because conventional propellers have essentially rigid blade-hub attachments, the aforesaid thrust dissymmetry occurs. The overall center of thrust of the propeller, in this condition, occurs 5-25% of blade radius, on the down side. It would be an advantage therefore to incorporate means within the propeller to permit flapping in a direction perpendicular to the original tip-path plane. This motion is referred to as beamwise flapping. Such a method and apparatus is provided in the present invention, wherein a hub flexure allows each blade to develop beamwise flapping velocities which reduce the cyclical angle of attack change and hence produces a more even thrust distribution around the propeller disc. While providing beamwise flexing, the assembly is relatively stiff within the plane of rotation and in blade torsion, which prevents in-plane instabilities and blade flutter. In this manner propulsive efficiency of fixed wing aircraft is greatly increased.